The annual energy output to be obtained by a wind turbine decisively depends, apart from the performance of the generator as installed, on the rotor diameter. Thus, for increasing the efficiency, it is desirable to use rotors of the largest possible size. However, when enlarging the rotor diameter while otherwise operating the plant under the same conditions, difficulties arise because the stresses acting on the rotor, the nacelle, the tower and the foundation will increase at least by the second power of diameter. Presently usual ratios between the performance of the generator as installed and the rotor area (rating) are in a range from 460 to 330 W/m2, the latter value pertaining to pitch-regulated turbines optimized for inland use.
According to an approach frequently used in wind energy technology, an existing turbine to be used in sites with weak winds can be retrofitted to have a larger rotor diameter, with the switch-off speed being lowered from e.g. 25 m/s to 20 m/s to safeguard that the stresses will remain in the allowable range.
Further, in plants with blade adjustment (pitch-type plants), it is a usual practice to adjust the rotor blades towards the direction of the feathered pitch already before the rated power is reached, thus reducing the stresses (particularly those acting on the tower).
According to a more complex and longer-known approach for reducing the above mentioned stresses, the rotational speed of the rotors and/or the power output of the turbine can be decreased in case of high wind velocities. For technical reasons (design of the transmission and/or generator and/or converter), decreasing the rotational speed of the rotors will have the effect that the power output is reduced at least according to the same ratio. Since, however—as widely known (cf. for instance “The Statistical Variation of Wind Turbine Fatigue Loads”, Riso National Laboratory, Roskilde DK, 1998)—the largest part of the high stresses that tend to shorten the lifespan will occur at high wind velocities, the above approach is successfully used particularly at inland locations for improving the efficiency of wind energy plants. Particularly at inland locations, use can thus be made of larger rotors which during the frequent low wind velocities will yield higher energy outputs but upon relatively rare high wind velocities will have to be adjusted correspondingly.
Further, the state of the art (DE 31 50 824 A1) includes an opposite approach for use in a wind turbine with fixed rotational speed, wherein, during high wind velocities with merely low turbulences, the power output of the turbine can supposedly be increased beyond the rated power by adjusting the rotor blade angle through evaluation of signals from a wind detector.